OP - Since you're after an 8x, I assume it's 3 stops you want.
If you're going for a Lee filter, be wary that they specify their filters in terms of optical density. So 0.3 = 1 stop, 0.6 = 2 stops, 0.9 = 3 stops etc.
The Lee Big Stopper (mentioned above) has an optical density of 10 (give or take) which equates to 10 stops. 10 stops means your shutter is open for 1000x longer than it would be without the filter... This can be a help or a hindrance.
On colour cast - up to 3 stops, colour cast is not much of a problem (except with Cokin) but my experience has been that all filters exhibit some colour cast. The solution is to make a test image using something like a X-rite Color Checker. As a rule, you can get away with somewhat higher ISO so the photographic process doesn't need to take forever.